Study of Photoexcited States and Nonlinear Optical Responses in Insulating Copper Oxides

Mott绝缘体中的电荷间隙源于强电子关联。因此，我们希望得到与带状绝缘体不同的光学响应。在……里面。本课题采用半填充Hubbard模型及其有效模型，从理论上研究了典型的Mott绝缘体--氧化铜绝缘材料中的光激发态和非线性光学响应。我们发现，与自旋和电荷分离的一维相比，系统中光掺杂载流子和局域自旋的相互作用在非线性光学响应中起着至关重要的作用。我们还发现，二维绝缘铜酸盐中双磁子喇曼散射的反常入射光子能量依赖是由自旋的存在引起的。为了阐明光激发态对维度的依赖关系，我们考察了线性光吸收光谱的温度依赖关系。在一维体系中，由于自旋-电荷的分离，在低温下不存在显著的温度依赖性。相反，在两个维度上发现了特征的温度依赖关系，这来自载流子和自旋的相互作用。非线性光学响应中不仅有光学允许态的贡献，也有禁忌态的贡献。为了考察这两个态的分布，我们对进行了动力学密度矩阵重整化群计算。一维哈伯德模型。我们发现，在真实材料中，大库仑相互作用中态的简并性被解除。

Charge gap in the Mott insulator originates from strong electron correlation. Therefore, we expect optical responses that are different from those in the band insulator. In. this project, we theoretically examine photoexcitaed states and nonlinear optical responses in insulating copper oxides that are typical Mott insulators by using a half filled Hubbard model and its effective modelWe investigated the nonlinear optical responses in the two-dimensional Mott insulator, applying the numerically exact diagonalization technique to the effective model. We found that, in contrast to one dimension where spin and charge are separated, the interplay of photo-doped carriers and localized spins in the system plays a crucial role in the nonlinear optical responses. We also found that anomalous incident-photon energy dependence of the two-magonon Raman scattering in two-dimensional insulating cuprates is caused by the presence of the spins. In order to clarify the dependence of photoexcited states on dimensionality, we examined the temperature dependence of the linear optical absorption spectrum. In one-dimensional system, the there is no remarkable temperature dependence at low temperature because of spin-charge separation. In contrast, characteristic temperature dependences are found in two dimensions, which come from the interplay of carriers and spins. Not only the optically allowed but also forbidden states contributes in the nonlinear optical responses. To examine the distribution of both the states, we performed a dynamical density-matrix renormalization group calculation for the. one-dimensional Hubbard model. We found that the degeneracy of the states seen at large Coulomb interaction is lifted in real materials.